Adaptive optic systems for correcting wavefront distortions in a light beam are well known in the art. A critical element of these systems is the wavefront sensor which detects the distortions. The distortions are generally caused by the wavefront passing through an atmospheric disturbance. The planarity of the wavefront is improved by correcting these distortions, which must first be accurately detected.
The apparatus labeled prior art in FIG. 1 herein, of U.S. Pat. No. 4,725,138 to Wirth et al., is a method for detecting the distortions. The apparatus is an optical centroid processor ("OCP") wavefront sensor which employs a filter array to detect and quantify a wavefront tilt.
Referring to FIG. 1, a light beam 30 from a distant object or the return beam from an object illuminated by a reference source is focused on the apparatus. The light beam 30 may be alternated in a beam combiner 32 with a reference beam 34, generally from a laser 36, forming a beam 38 which is imaged on a lenslet array 40. The lenslet array divides the combined beam 38 into a plurality of subaperture beams 42 which are focused on an image intensifier 44. The subaperture beams are amplified and focused on a collimator 46. The collimated beams 48 are impinged on a pair of lenses 50,52 by a beam splitter 54. The first lens 50 focuses subaperture images 55 on each cell 56 of a filter array 58 forming a pixel spot on each cell, the circumference of which is shown by a line 60 (FIG. 1A). A filtered subaperture image 62 is impinged on each corresponding cell 64 of a photodetector array 66. The detector array comprises a plurality of photodiodes or other devices which convert photon energy to electric energy. The second lens 52 focuses a subaperture image 68 directly on each cell of a reference photodetector array 70.
Each mask cell 56 comprises a plurality of opaque chevrons 72 (FIG. 1A) which are wider on one end and decrease in the positive x-direction, and a plurality of transparent chevrons 74 which are wider on the other end and decrease in the negative x-direction. Whenever there is a tilt in the subaperture wavefront in the x-direction, the spot, indicated by a dashed line 60a, from the subaperture beam 55 is shifted from the center of the mask cell, thereby increasing the intensity of the light through the cell to the photodiode as compared the center spot, assuming the same intensity. A corresponding pixel spot (i.e. a shifted spot when there is a tilt and an un-shifted spot when there is not) is also measured without passing through a filter array on the reference photodetector array 70. The amount of light to the photodiode directly corresponds to the electrical output of the photodiode.
Thus, for a particular pixel cell, the photodiode output is compared to the reference photodiode output of the corresponding cell to determine the amount of shift in the spot. A ratio calculator 76 is connected to the photodetector arrays 66,70 by a pair of lines 78,80, to perform calculations based on a predetermined algorithm. The shift in spot position represents the tilt in a portion of the wavefront in the x-direction. The information obtained from the ratio calculator is used to correct the tilt in the wavefront of the original light beam by adjusting a deformable mirror (not shown).
The OCP sensor also provides for detecting the wavefront tilt on two axes as described in an alternate embodiment of the apparatus in the '138 patent. Referring now to FIG. 2 herein, the image impinged on the collimator 82 is achieved in the same manner as previously described. The collimator focuses three identical images 84 on three separate identical lenses 86,88,90. The first lens 86 focuses a subaperture image 92 on each cell 93 of a first filter array 94, which consists of mask cells 93 (FIG. 2B) comprising a plurality of opaque chevrons 98 and a plurality of transparent chevrons 100 in the x-plane. A pixel spot, the circumference of which is shown by a line 102, is focused on the center of the mask cell when this portion of the wavefront is without a tilt. When this portion of the wavefront is tilted, a shifted spot, shown by a dashed line 102a, is produced. The intensity of the light 103 through the x-direction cell 93 is detected by the corresponding photodiode in the first photodetector array 104 (FIG. 2).
The second lens 88 focuses a plurality of subaperture images 105 directly on a second photodetector array 106. The third lens 90 focuses a plurality of subaperture images 108 on a second filter array 110, which comprises an array of mask cells 112 (FIG. 2C), each cell having a plurality of opaque chevrons 114 and a plurality of transparent chevrons 116 in the y-plane. A pixel spot, the circumference of which is shown by a line 118, is focused on the center of the mask cell when this portion of the wavefront is without a tilt. When this portion of the wavefront is tilted a shifted spot, shown by a dashed line 118a, is produced. The intensity of the light through the y-direction cell 112 is detected by the corresponding photodiode in the third photodetector array 122. An output signal of each of the corresponding photodiodes for the x-direction and each of the photodiodes for the y-direction is compared to the corresponding photodiode output signal in the second photodetector array by the ratio calculator. Thus, wavefront tilt can be detected on two axes.
Generally, the OCP system described in patent '138 employs two-dimensional photodetector arrays. The two-dimensional arrays are employed because individual photodiodes or one-dimensional photodetector devices would require a complex mechanical system to optically align the photodiodes.